Combustible gas detectors can be divided into two general categories. The first category includes a variety of “passive” technologies of which the electrocatalytic (catalytic bead) type is the most common. The second category is based on technology that uses infrared absorption as the detection modality.
Infrared (IR) absorption is considered “active” since an IR source emits a signal, usually many times a second, and the amount of energy falling on the detector serves as an active measure of the gas concentration at that moment. Any failure of the source or detector, or blockage of the signal by dirt, is detected immediately as a malfunction. For this reason, IR detectors are also considered to be fail-to-safe. IR gas detectors can be used for “point” (single location) or “open path” (line of sight) applications.
Electrocatalytic or “catalytic” detectors have been used for over 30 years and are widely deployed in a variety of industries as single-point detectors for combustible gases. They function on the relatively simple and reliable principle that a combustible gas can be oxidized to produce heat. The resulting temperature change can be converted, via a standard Wheatstone bridge, to a sensor signal. That signal can then be used to activate alarms and initiate fire preventative action. Even though these detectors can be manufactured to be very low cost, their primary drawback is that they can be contaminated or “poisoned” and they hence require dedicated user attention.
An alternative method of measuring gas concentration is based on absorption of infrared (IR) radiation at certain wavelengths as the optical signal passes through a volume of gas. Devices using this technology have a light source and a light detector and measure the light intensity at two specific wavelengths, one at an absorption (active) wavelength and one outside of the absorption (reference) wavelength. If a volume of gas passes between the source and detector, the amount of light in the active wavelength falling on the detector is reduced, while the amount of light in the reference wavelength remains unchanged. Much like the catalytic detectors, the gas concentration is determined from the relative difference between the two signals.
Generally, there are several key advantages to using IR-based detectors: immunity from chemical poisons; does not need oxygen or air to detect gas; can work in continuous exposure gas environments; fail-to-safe technology; and internal compensation can eliminate span drift.